Feeding strategies of earthworms and their influence on soil processes are often inferred from
morphological, behavioral and physiological traits. We used 13C and 15N natural abundance in earthworms,
soils and plants to explore patterns of resource utilization by different species of earthworms in three
tropical ecosystems in Puerto Rico. In a high altitude dwarf forest, native earthworms Trigaster longissimus
and Estherella sp. showed less 15N enrichment (delta 15N = 3–6%) than exotic Pontoscolex corethrurus (15N =
7–9%) indicating different food sources or stronger isotopic discrimination by the latter. Conversely, in a
lower altitude tabonuco forest, Estherella sp. and P. corethrurus overlapped completely in 15N enrichment
(delta15N = 6–9%), suggesting the potential for interspecific competition for N resources. A tabonuco forest
converted to pasture contained only P. corethrurus which were less enriched in 15N than those in the forest
sites, but more highly enriched in 13C suggesting assimilation of C from the predominant C4 grass. These
results support the utility of stable isotopes to delineate resource partitioning and potential competitive
interactions among earthworm species. Copyright # 1999 John Wiley & Sons, Ltd.

Investigation of single or mixed assemblages of
native Estherella sp. and exotic Pontoscolex corethrurus
from a rain forest in Puerto Rico was undertaken to understand
resource use patterns, and linkages with C and N
mineralization in a 19-day incubation. Resource use was
explored with addition of 15N-enriched leaf litter and 13Cenriched
glucose to reconstructed organic and mineral soil
horizons. Juvenile Estherella sp. became at least 6.06‰
more enriched in 13C than sub-adult Estherella sp. or adult
P. corethrurus. Sub-adult Estherella sp. became >3.6‰
enriched in 13C over P. corethrurus. δ15N acquired by P.
corethrurus was greater by 0.83–1.56‰ in the mixed-species
than the single-species assemblages. δ15N of subadult
Estherella sp. was enriched by 0.73–0.81‰ over juvenile
Estherella sp. in the single-species assemblage. Net
N immobilization occurred in the organic layer of all 15Nenriched
treatments. Net N mineralization in mineral soil
layers was significantly greater in microcosms with P.
corethrurus than in those containing only Estherella sp..
Cumulative respiration was greatest in P. corethrurus assemblages,
however, assemblages with only Estherella sp.
released more 13C in respiration. P. corethrurus assimilated
different N resources when incubated with, as compared
to without, native Estherella sp.. δ13C and δ15N signatures
acquired by assimilation of 13C and 15N differed
by species, developmental stage, and competitive interactions.
The results showed that alone, exotic P. corethrurus
induced higher mineralization rates than native Estherella
sp., but that the interaction of exotic and native species
impinged on resource use by P. corethrurus, reducing the
effect of the exotic species on C and N mineralization. Invasion
of exotic P. corethrurus may change the mineralization
potentials of C and N and their biogeochemical cycling
in soils.